1. Technical Field
An acoustic wave filter device having a pass band lower than a trap band is disclosed.
2. Description of the Related Art
Up until now, various filter devices using acoustic waves, such as surface acoustic wave devices and boundary acoustic wave devices, have been proposed as band filters for communication apparatuses. For example, surface acoustic wave devices have been widely used as RF band filters for mobile phones.
Recently, digital terrestrial television broadcasting has been becoming widespread. In digital terrestrial broadcasting, one channel is divided into 13 segments. Of the 13 segments, one segment located at the center of one channel is used as a transmission band for mobile phones. Broadcasting using this one segment is so-called one-segment broadcasting, which uses a transmission band from 470 MHz to 770 MHz.
Transmission bands for mobile phones may vary among different methods and communications companies. For example, a band of 800 MHz or more, such as from 830 MHz to 845 MHz or from 898 MHz to 924 MHz, a 1.7 GHz band, and a 1.9 GHz band are used. Some mobile phones are capable of receiving and even recording one-segment broadcasting. In such a mobile phone, if phone transmission is performed simultaneously with reception and recording of one-segment broadcasting, a received video image may be distorted due to the effect of transmitted radio waves. Accordingly, there is a demand for band rejection filters having a trap band equal to a transmission band of the mobile phone and having a pass band lower than the trap band.
Japanese Unexamined Patent Application Publication No. 2004-129238 (“the '238 application”) discloses an example of a surface acoustic wave filter device of band rejection type. FIG. 14 illustrates a circuit configuration of the surface acoustic wave filter device described in the '238 application.
As shown in FIG. 14, a surface acoustic wave filter device 501 has an input terminal 502 and an output terminal 503. An inductor 504 is provided in a series arm connecting the input terminal 502 and the output terminal 503. A surface acoustic wave resonator 505 is connected between one end of the inductor 504 and a ground potential, while a surface acoustic wave resonator 506 is connected between the other end of the inductor 504 and the ground potential. That is, the inductor 504 and the two surface acoustic wave resonators 505 and 506 are provided as a π-type filter circuit.
In the surface acoustic wave filter device 501, resonance frequencies of the surface acoustic wave resonators 505 and 506 are placed in an attenuation band in intended filter characteristics, an electric signal at the resonance frequencies is lowered to the ground potential, and thus attenuation characteristics are obtained. That is, a trap band is defined by the resonance frequencies of the surface acoustic wave resonators 505 and 506.
In the surface acoustic wave device 501, the resonance frequencies of the surface acoustic wave resonators 505 and 506 are placed in a frequency range where a trap is provided, and thus the trap band is defined. However, until the electric signal at the resonance frequencies is lowered to the ground potential, the electric signal has a common inductance component on a piezoelectric substrate or a package included in the surface acoustic wave filter device 501. As a result, since the signal leaks through the surface acoustic wave resonators 505 and 506, satisfactory attenuation characteristics cannot be achieved in the trap band. Therefore, for example, if the surface acoustic wave filter device 501 is used as a band rejection filter for a reception stage of one-segment broadcasting in the mobile phone capable of receiving one-segment broadcasting, it is difficult to reliably attenuate transmitted radio waves of the mobile phone during reception or recording of one-segment broadcasting.